Russia’s meteorological service has reported “extremely high pollution” of a radioactive isotope in the Urals near a facility that previously suffered the third-worst nuclear catastrophe in history.

The news bolsters international reports that a ruthenium-106 leak originating in the Urals sent a radioactive cloud over Europe. Greenpeace Russia has said it will ask the prosecutor general to investigate the possible cover-up of a nuclear accident.

Rosatom, Russia’s state nuclear company, has denied the leak. Its Mayak facility, where an explosion in 1957 contaminated a swath of central Russia, told state news agency RIA Novosti it had not processed nuclear fuel with ruthenium-106 this year.

The isotope, which doesn’t occur naturally, was detected in Germany, Austria, Italy and Switzerland in late September. France’s nuclear safety institute said the “major” radiation leak likely occurred between the Urals and the Volga river.